This invention relates to a dual-polarization planar antenna for use in satellite communication systems and radio communication systems in a microwave band.
In satellite communication systems in a microwave band, it is necessary to switch vertical and horizontal polarizations for every reception channel. Also in radio communication systems, transmission and reception are efficiently carried out by switching vertical and horizontal polarizations or clockwise and counterclockwise circular polarizations. In this connection, development has been made of an antenna operable with controllably variable polarizations.
As a planar antenna of the type described, a microstrip antenna is known. Referring to FIG. 1, the microstrip antenna comprises a ground conductor 1 and a radiation patch element 3 of a square shape. The ground conductor 1 has a slot 12 formed at a position right below the radiation patch element 3. A triplate line is formed by a combination of the ground conductors 1 and 11 and a feeding line 8. The triplate line and the radiation patch element 3 are electromagnetically coupled to each other through the slot 12. A feeding line 4 is connected to one end of the radiation patch element 3. The radiation patch element 3 is excited by the feeding lines 4 and 8 in a first excitation direction A and a second excitation direction B, respectively. The first and the second excitation directions A and B are perpendicular to each other. With this structure, it is possible to use both the vertical and the horizontal polarizations. Such an antenna is disclosed in the paper entitled "Study on Dual-Polarization Planar Antenna" and prepared for the 1990 Springtime National Conference of Electronics, Information, and Communication Society, Japan, Paper No. B-133, and the paper entitled "Radiation Characteristics of Dual-Polarization Planar Array" and prepared for the 1990 Autumnal National Conference of Electronics, Information, and Communication Society, Japan, Paper No. B-93.
The above-mentioned dual-polarization microstrip antenna has a switching circuit for electrically switching the outputs of the feeding lines 4 and 8. Accordingly, when the antenna is operated with the vertical and the horizontal polarizations having polarization planes perpendicular to each other, it is possible to obtain a desired polarization output without mechanical rotation of the antenna itself. In addition, the dual-polarization microstrip antenna can rapidly follow the change of the polarization plane. As a result, interruption of communication is avoided. A mounting structure is simple because a mechanical drive is unnecessary.
In the above-mentioned conventional antenna, the triplate line formed by a combination of the ground conductors 1 and 11 and the feeding line 8 is electromagnetically coupled to the radiation patch element 3 through the slot 12. In this event, a parallel plate mode wave is produced and propagated between the ground conductors 1 and 11 to cause leakage of electric power. This results in occurrence of unnecessary coupling or radiation to thereby deteriorate the characteristic of the antenna. Such phenomenon is described in Proceedings of ICAP89, April, pp. 346-368 (1989), Digest IEEE International Microwave Symposium, pp. 199-202 (1988), A.P91-35 "Analysis and Solution of the Parallel Plate Mode by the Use of the Spatial Circuit Network Method", and other reports in recent conferences.